It sounds like you are booting Windows XP and then trying to launch the Vista installation. Unfortunately that's not an option since you have the 32-bit edition of XP and want to install Vista x64. The solution is to insert the Vista DVD, reboot your computer, and, when prompted, press any key to boot from DVD. If you are not prompted to 'press any key to boot from CD/DVD,' you will need to edit your system BIOS such that your DVD drive is the primary boot device. You can then proceed as normal.

Also, note that you cannot upgrade from XP x86 to Vista x64 due to the same 32/64 bit issue, so you'll either have to install Vista x64 onto another partition or wipe the drive, including any files and settings on it, to perform a clean install of Vista.

If you want a simple upgrade you'll have to use the 32-bit version instead.

John